Amplifier circuit



A ril 25, '1939. v L M CRAFT 2,155,404

AMPLIFIER CIRCUIT Filed Dec. 26, 1935 2 Sheets-Sheet 2 L 1144 MORGAN 0MP)? 6 IN V EN TOR.

a 6 ATT'ORNEY -Patentecl Apr. 25, 1939 UNITED STATES PATENT OFFICE AMPLIFIER CIRCUIT.

Application December 26, 1935, Serial No. 56,266

1 Claim.

My invention relates broadly to electrical amplifier circuits and more particularly to an arrangement of amplifier circuit for eliminating or reducing retroactive currents due to capacity coupling between electrodes of the electron tubes.

One of the objects of my invention is to provide means for overcoming or neutralizing the effect of capacity coupling between electron tube electrodes to the end that a greater amplification of input power may be obtained without resulting instability of operation.

It is a well known fact that in the three element vacuum tube, such coupling cannot be avoided due to the mutual capacity of the elements of the structure. According to my invention, I induce a voltage in the output circuit of the tube due to mutual induction between the input and output circuits in such a manner as to compensate for the current which flows between the grid and plate due to this mutual capacity.

Another object of my invention, therefore, is to provide means for inducing a voltage'in the plate circuit of a three-electrode amplifier tube to produce a current in opposition to the current passed by the inter-electrode grid-plate capacity in the amplifier tube, whereby the efiect of the grid-plate capacity coupling is neutralized.

A further object of my invention is to provide means for inducing a voltage in the plate circuit of an amplifier tube from a current in thegrid circuit by mutual induction, the resulting current in the plate circuit being effective to neutralize the current due to the grid-plate capacity coupling in the amplifier tube.

Still another object of my invention is to provide inductances in the grid and plate circuits of a three electrode amplifier tube coupled in such a manner that voltage induced in the plate inductance is connected with the plate electrode in the same polarity as the signal voltage impressed upon the grid electrode, that is, in a counterregenerative sense; the current resulting from th induced voltage being effective to neutralize the current due to the grid-plate capacity coupling in the amplifier tube.

A still further object of my invention is to provide means for varying the mutual inductance of inductances connected in the plate and grid cirsuits of an electron tube as set forth in the preceding paragraph, the mutual inductance being varied simultaneously with the tuning of the plate or the grid circuit, whereby the neutralization is complete at all frequencies received.

My invention will be more fully understood from the specification hereinafter following by reference to the accompanying drawings, in which:

Figure 1 is a schematic circuit diagram of a three element vacuum tube connected as an amplifier with mutual inductance between input and output circuits in accordance with my invention; Fig. 2 is a simplified diagram of the impedance elements of the alternating current network illustrated in Fig. 1; Fig. 2a is a theoretical graph employed in connection with Fig. 2 to illustrate the theory of operation of the system of my invention; Fig. 3 is a circuit diagram of a modified form of my invention; Fig. 4 is a circuit diagram of a further modified form of my invention; and 5 Fig. 5 illustrates one form of mechanism which, may be employed in connection with the system shown in Fig. 3 or Fig. 4, for varying the inductive relation of a pair of inductance coils.

The circuit arrangement of my invention may be applied to amplifiers having various types of input circuits, as, for example, an untuned transformer secondary, or condenser feed to gridv with means for. supplying proper bias. I may use a tuned circuit C1---L1 connected to the input of the amplifier as shown in Fig. v1 with advantage. Inductance L2 and condenser C2 form the output circuit, with mutual inductance M between input and output, with a coefiicient of coupling substantially less than unity and in a positive sense. By coupling in a positive sense, I mean that species of coupling between the two coils which, if acting alone, would cause the plate 10 and grid g to be at the same polarities with respect to the cathode c of the amplifier electron tube T. The grid-plate inter-electrode capacity is represented at Cgp in Figs. 1 and 2.

If a current I1 is assumed flowing in L1, then referring to Fig. 2, E1 is equal to I1wL1 and.

where E1 and E2 are the voltages acting around the circuit in a clockwise direction. Assuming temporarily that C2 is replaced by a short circuit, the current through the short circuit due to the voltage E1 will be which leads E1 in phase. Also, the current through the short circuit due to the voltage E2 will be an 101 L2 which lags E2 90 in phase. The voltages E1 and E2 are out of phase due to the fact thatEz '55 currents produced by the two voltages are in.

opposition.

The current in the short circuit, then, is

Ic -Iz and will be zero when ha -=11 or M=w cgpL1L2- V w=21r times the frequency of I1. The above description can perhaps be better understood by reference to Fig. 2a. which indi-.

cates theoretically the phase relations of the voltages and currents considered with respect to the assumed short circuit, the curve representing each being clearly'designa'ted on the drawings; in connection with Fig. 2a. Line mac gives, at its intersection with the several curves, 'th'eoreti'cal instantaneous values-of the various currents'and voltages, and indicates that the "currents-le and 111. as represented, are equal an d fiowing oppositedirections. The'resultant current,';or that which would flow in theshort circuit assumed, is, therefore, zero.

' As" there willbe no current in the short circult, itmay be replaced by any value of C2 and no voltage will appear acrossfit and nocirc'ulatmfg current now'in I.z" oi;' This neutralization is complete at only one frequency but gives a workable amplifier overa'two to one frequency In order to' increas'ethe effectiveness of the neutralizing system of my invention over a wider frequency range, an arrangement suchfas'that shown in Fig 3 may be employed. The circuit illustrated diagrammatically in 3'is substaziti'ally identical with thatshowniin Fi 1 and compris'es'an input circuit including inductance L1 and: condenser (11- connected with the gr'idlg, and an output circuit..includi-ng inductance L2 and condenser 'Caconnected with thepl'ate p, of the amplifier electron tubeT which'also includes the cathode c. The 'inductances'Li. and Lz are coupled, as. in the circuit showhin Fig. 1; witha mutual inductance of M, which, however, in the system illustrated in Fig. 3, is variable and controlled simultaneously with the tuning of the output circuit through the action of a cam element e. From the equation M=tu cgpL1L2,. where w=2'1rf, it is clear that if the mutual inductance M is varied'in' proper proportions as the'frequency f is varied, neutralization is effectedover the'entire frequency range covered; Also, according to my invention, the mutual inductance may be varied simultaneously with the tuning." of the input'instead of the output circuit... 7 In Fig. 4, I indicate a further modification of the system of my invention wherein'portions only of the inductance in the input and output circuits are coupled. Inductances l; and 3 are coupled-with mutual inductance M, and the inductance relation thereof is variable in connection with the variation of the tuning of the input circuit (or the output circuit) through the cam element e. The tuning in the modified circuit of Fig. 4, is effected by varying both the inductance and the capacitance of thecircuiti L1 and "end 1 01 in the input circuit; and L2 and C2 in-the output circuit.

Fig. 5 illustrates a mechanism adaptable to the systems shown in Figs. 3 and 4 for varying the inductive relation of a pair of coils in connection with a tuning element such as a variable condenser. The mechanism shown in Fig. 5 comprises a pair of inductance coils i1 and i2 arranged in variable inductive relation, the coil i1 being movable with respect to the coil i2. The coil i1 is mounted on a support (1, connected with a bar a which is slidably mounted in supporting guides At the lower end of the support d is a pin or roller 7' engaging the eccentric slot e in-the rotatable element m. A variable tuning condenser C has its shaft 8 connected through a 'coupling'k and a flexible drive shaft ,f to the rotatable element m so that as the tuning condenser is varied the inductive relation of the coils i1 and i2 is varied in accordance with the contour of the eccentric slot or cam slot e. It is noted from the equation M =w cgpL1LZ that M must vary in proportion to the square of the frequency; and the contour of the eccentric slot e is devised to provide for such relative variation of the mutual inductance. The slot 6 shown in Fig. 5 -ismerely illustrative of the position and function of the slot in the mechanism, and is not an=accurate trace of the particular form of slot adaptableto the system of my invention.

The circuit arrangement of my invention has numerous advantages among which are the inexpensive assembly possible, by use of the circuit and the simplicity of the circuit. As it is ordinarily necessary to shield the input and output input circuit and the 'output circuit in such a manner as to secure the advantages already stated and explained. The invention is not limited to the specificcircuit details illustrated in the accompanying drawings, as mutual inductionmay occur only between portions of L1 and L2, as suggested in Fig. 4.

The circuit arrangement of my invention is particularly adapted for eliminating or substantially reducing electrode capacity effects in transmitting amplifiers without the employment of auxiliary neutralizing circuits which add cost and complication to the equipment.

I While I have described my invention in one of its preferred embodiments, I desire that it be understood that modifications can be made and that no limitations upon my invention are intended except as may be imposed by the scope of the appended claim. What I claim as new and desire to secure by Letters Patent of' the United States is as follows: An electron tube amplifier circuit comprising an electron tube having anode, control grid, and cathode electrodes with inherent capacity (Cgp) between said grid and plate electrodes, a tunable input circuit connected with said control grid and cathode electrodes'a tunable output circuit connected with said anode and cathode electrodes, Variable capacitive elements in said input and output circuits for tuning said circuits inductive elements (L1 and L2) in said input and output circuits providing substantially the entire inductance in said circuits and being arranged with mutual inductance therebetween, and means operative in conjunction with one of said tunable circuits for varying said mutual inductance, said means being adapted to cause the coefiicient of said mutualinductance (M) to vary as the square of the frequency (f) to which said tunable circuit is tuned, in accordance with the relation M=(21rf) CgpL1L2, said coeflicient of mutual inductance being substantially less than unity and effective in a positive, non-regenerative sense.

LIVA MORGAN CRAFT. 

